The present invention relates to an image projector for optically projecting a document image onto a screen and an image forming apparatus for image projection and, more particularly, a plain paper projector (PPP).
It is a common practice with a purely optical overhead projector (OHP) or a slide projector to use a transparent sheet, i.e., an OHP sheet or a slide film, as taught in Japanese Patent Laid-Open Publication No. 4-128734 by way of example. Light transmitted through the transparent sheet and, therefore, an image carried on the sheet is projected onto a screen in an enlarged scale. This kind of projector, however, cannot project opaque plain sheet documents, not to speak of thick book documents. It follows that the images of such documents have to be written on transparent sheets by hand or transferred by a copier.
In light of the above, a projector may be constructed to project light transmitted through an electrically driven TFT (Thin Film Transistor) type liquid crystal display, as proposed in, for example, Japanese Patent Laid-Open Publication No. 4-149417. Such a semioptical and semielectrical projector photoelectrically converts a document image and then electrically writes it in the liquid crystal display. The resulting image written to the display is optically projected onto a screen in an enlarged scale. Since this projector transforms the image to an electric signal beforehand and stores it, it is not necessary for the operator to replace the document. However, a problem with the projector using a TFT type liquid crystal display is that the display, which is electrically driven, increases the size and cost of a scanner for scanning a document and those of a digital image processing device. Another problem is that this type of projector is troublesome to operate. Moreover, since the TFT type display needs TFTs for respective pixels, it cannot be easily increased in size due to limited yield.